An increase in the capacity of information communication has been realized by a wavelength-multiplexing optical transmission system. The wavelength-multiplexing optical system is called Wavelength Division Multiplexing (WDM) system as well. In recent years, the capacity of information transmitted and received by one terminal increases according to improvement of the performance of communication terminals such as a smartphone. There is a request for a further increase in the capacity of the information communication. For the purpose of realizing improvement of reliability for communication paths and communication devices, there is an increasing request for network flexibility for optimizing a network configuration according to demands. To meet these requests, a multi-way WDM system adapted to a flexible grid specified in Non Patent Literature 1 is put to practical use. In the multi-way WDM system adapted to the flexible grid, it is possible to change bandwidths of wavelengths.
In the WDM system, deterioration of an optical transmission characteristic is prevented by a function of adjusting an optical level of each of the wavelengths to a specified value. This function is called, for example, dynamic gain control as well. As an example of the conventional WDM system, Patent Literature 1 describes a WDM optical transmission system that adjusts an optical level on a transmission side for each of wavelengths such that a signal characteristic at a reception end is optimized.
As influence due to level fluctuation, when the optical level rises, the influence of a nonlinear phenomenon increases and, when the optical level falls, an optical signal to noise ratio (OSNR) is deteriorated. These deteriorate the signal characteristic. A transmission distance is reduced by the signal characteristic deterioration.